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For A2(g) -> B2(g) + C2(g) the equilibrium constant Kp = 9 atm at a total pressure of 7 atm and 300 K. Given molar masses A2 = 70, B2 = 49, C2 = 21 g/mol, find the average molar mass of the equilibrium mixture.

1. 50
2. 45
3. 40
4. 37.5

Correct answer: 40

Solution

Mass is conserved while moles increase from 1 to (1+alpha), so M_avg = 70/(1+alpha); with alpha = 0.75 this gives 40 g/mol.

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